1. Field of the invention
The present invention relates to a magnetic recording/reproducing apparatus which can control a running speed and tension of magnetic tape constant, and more particularly to a magnetic recording/reproducing apparatus of so-called reel-to-reel type employing no capstan.
2. Description of the Background Art
For precise recording/reproducing with high density in a magnetic recording/reproducing apparatus employing magnetic tape, as well as maintaining running speed of the tape constant, the tension applied to the magnetic tape must be maintained constant.
Therefore, in a conventional magnetic recording/reproducing apparatus, the running speed of the magnetic tape is kept constant by pressure-driving the magnetic tape using a capstan and a pinch roller, for example. The tension applied to the magnetic tape is controlled to be constant by a tension detection mechanism called a tension arm.
A schematic structural diagram of a conventional magnetic recording/reproducing apparatus is shown in FIG. 5.
Referring to FIG. 5, in conventional magnetic recording/reproducing apparatus, the running speed of magnetic tape 2 is controlled by a capstan or the like, and the tension is controlled to be constant by a mechanism employing a tension arm 60 or the like. Accordingly, when the running speed of magnetic tape 2 becomes high, the running speed can not be precisely controlled, and resonance is produced between the magnetic tape 2 and the tension arm 60. As a result, there are problems that the tension applied to the magnetic tape 2 can not be controlled to be constant and the entire structure becomes complicated.
Then, a so-called reel-to-reel type magnetic recording/reproducing apparatus in which a magnetic tape 2 is directly moved from a reel 3 to a reel 4 without employing a capstan has been devised. In this reel-to-reel type magnetic recording/reproducing apparatus, the running speed of magnetic tape 2 is controlled by detecting rotation of a rotation type speed detection roller or reels 3, 4. That is, feed back control is performed by detecting the rotational cycle, finding an error between a predetermined cycle and the same, amplifying the same, and providing the same to a reel driving motor. The tension applied to the magnetic tape 2 is kept constant by detecting a tape tension and supplying corresponding torque to a supply reel.
However, the above-described reel-to-reel type magnetic recording/reproducing apparatus has the following problems.
That is, because of the resonance between reels 3, 4 and magnetic tape 2, a gain of a control loop can not be set high. Accordingly, highly precise control can not be performed.
Next, the reason why resonance is produced between reels 3, 4 and magnetic tape 2 will be described.
The two reels 3, 4 with the tape 2 laying across them constitute a spring system (reels 3, 4 can be regarded as inertia and tape 2 as a spring). Accordingly, especially when reel 3 on one side only is controlled, since torque is applied only to one inertia, expansion and contraction of the spring (tape) 2 occurs to produce resonance.
Furthermore, if there is a tension arm 60 as shown in FIG. 5, a spring system also exists between the arm 60 and tape 2. As a result, as described above, resonance is very likely to be produced because external vibration due to unevenness of reel radius or the like increases especially in high speed tape feeding.
FIG. 6 is a graph showing a frequency characteristic of an open loop transfer function of a magnetic tape running system of a reel-to-reel type magnetic recording/reproducing apparatus which controls the running speed by detecting a running speed of magnetic tape and feeding back the same to a take-up reel.
As shown in FIG. 6, there exists resonance by reels 3, 4 and magnetic tape 2, the phase lags by more than 180.degree. at this point, so that the gain can not be set high for stabilizing the control system. Therefore, the servo system response frequency is limited low, and also with a low gain in a low frequency region, it is difficult to keep deviation small.